Merge branch 'migrate_session_expire_delay_unless_it_exists' into 'master'

Check if session_expire_delay column exists before adding the column.

Fixes #2052 

Fixes #1985

See merge request !1029

db/migrate/20150609141121_add_session_expire_delay_for_application_settings.rb

 class AddSessionExpireDelayForApplicationSettings < ActiveRecord::Migration
   def change
+    unless column_exists?(:application_settings, :session_expire_delay)
       add_column :application_settings, :session_expire_delay, :integer, default: 10080, null: false
     end
+  end
 end